The present invention relates to a liquid crystal display device, especially, to a liquid crystal display device with the viewing angle dependency of the display screen abated by a combination of a liquid crystal display element and an optical retardation compensator plate.
Conventionally, liquid crystal display devices incorporating nematic liquid crystal display elements have been in widespread use for numeral-segment-type display devices such as watches and calculators, and recently the applications are finding more places with word processors, notebook-type personal computers, liquid crystal televisions mounted in automobiles, etc.
Generally, a liquid crystal display element has a transparent substrate, electrode lines for turning on and off pixels, and other components formed on the substrate. For example, in an active-matrix type liquid crystal display device, active elements, such as thin-film transistors, are formed on the substrate together with the electrode lines as switching means for selectively driving pixel electrodes by which voltages are applied across the liquid crystal. Moreover, in liquid crystal display devices capable of color display, color filter layers having colors such as red, green and blue are provided on the substrate.
Liquid crystal display elements such as the one mentioned above adopt a liquid crystal display mode that is suitably selected depending on the twisted angle of the liquid crystal: some of well-known modes are active-driving-type twisted nematic liquid crystal display mode (hereinafter, referred to as the TN mode) and the multiplex-driving-type super-twisted nematic liquid crystal display mode (hereinafter, referred to as the STN mode).
The TN mode displays images by orientating the nematic liquid crystal molecules to a 90xc2x0-twisted state so as to direct rays along the twisted directions. The STN mode utilizes the fact that the transmittance is allowed to change abruptly in the vicinity of the threshold value of the applied voltage across the liquid crystal by expanding the twist angle of the nematic liquid crystal molecules to not less than 90xc2x0.
The problem with the STN mode is that the background of the display screen sustains a peculiar color due to interference between colors because of the use of the birefringence effect of liquid crystal. In order to solve this problem and to provide a proper black-and-white display in the STN mode, the application of an optical retardation compensator plate is considered to be effective. Display modes using the optical retardation compensator plate are mainly classified into two modes, that is, the double layered super-twisted nematic optical-retardation compensation mode (hereinafter, referred to as the DSTN mode) and the film-type optical-retardation compensation mode (hereinafter, referred to as the film-addition mode) wherein a film having optical anisotropy is provided.
The DSTN mode uses a two-layered construction that has a display-use liquid crystal cell and a liquid crystal cell which are orientated with a twist angle in a direction opposite to that of the display-use liquid crystal cell. The film-addition mode uses a construction wherein a film having optical anisotropy is disposed. Here, the film-addition mode is considered to be more prospective in the standpoint of light weight and low costs. Since the application of such an optical-retardation compensation mode makes it possible to improve black-and-white display characteristics, color STN liquid crystal display devices have been achieved that enable color display by installing color-filter layers in STN-mode display devices.
The TN modes are, on the other hand, classified into the Normally Black mode and the Normally White mode. In the Normally Black mode, a pair of polarizer plates are placed with their polarization directions in parallel with each other, and black display is provided in a state where no ON voltage is applied across the liquid crystal layer (OFF state). In the Normally White mode, a pair of polarizer plates are placed with their polarization directions orthogonal to each other, and white display is provided in the OFF state. Here, the Normally White mode is considered to be more prospective from the standpoints of display contrast, color reproducibility, viewing angle dependency, etc.
However, in the TN-mode liquid crystal display device, liquid crystal molecules have a refractive index anisotropy xcex94n, and are orientated so as to incline to the above and below substrates. For these reasons, the viewing angle dependency increases: i.e., the contrast of displayed images varies depending upon the direction and angle of the viewer.
FIG. 11 schematically shows the cross-sectional construction of a TN liquid crystal display element 31. This state shows liquid crystal molecules 32 slanting upward slightly as a result of application of a voltage for halftone display. In such a liquid crystal display element 31, a linearly polarized ray 35 passing through the surfaces of a pair of substrates 33 and 34 along the normals thereto, and linearly polarized rays 36 and 37 passing through those surfaces not along the normals thereto cross the liquid crystal molecules 32 at different angles. Besides, the liquid crystal molecules 32 have a refractive index anisotropy xcex94n. Therefore, the linearly polarized rays 35, 36 and 37, upon passing through the liquid crystal molecules 32 in different directions, produce ordinary and extraordinary rays. The linearly polarized rays 35, 36 and 37 are converted to elliptically polarized rays according to the phase difference between the ordinary and extraordinary rays, which cause the viewing angle dependency.
In addition, in an actual liquid crystal layer, the liquid crystal molecules 32 show different tilt angles in the vicinity of the midpoint between the substrates 33 and 34 and in the vicinities of the substrates 33 and 34. The liquid crystal molecules 32 are twisted by 90xc2x0 around the normal.
For those reasons described so far, the linearly polarized rays 35, 36 and 37 passing through the liquid crystal layer are affected by the birefringence effect in various ways depending upon, for example, the directions and the angles thereof, resulting in complex viewing angle dependency.
Such viewing angle dependency can be observed, as examples, in the following situations. If the viewing angle increases from the normal to the display screen in the standard viewing direction, i.e. downward, and exceeds a certain angle, the displayed image has a distinct color (hereinafter, referred to as the coloration phenomenon), or is reversed in black and white (hereinafter, referred to as the tone reversion phenomenon). If the viewing angle increases from the normal in the opposite viewing direction, i.e. upward, the contrast decreases abruptly.
The aforementioned liquid crystal display device has another problem that the effectual range of viewing angle narrows with a larger display screen. When a large liquid crystal display device is viewed from a short distance in the front thereof, the same color may appear different in the uppermost and lowermost parts of the large screen due to the effect of the viewing angle dependency. This is caused by a wider range of viewing angle required to encompass the whole screen surface, which is equivalent to a viewing direction which is increasingly far off center.
To restrain the viewing angle dependency, Japanese Laid-Open Patent Application NO. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest that an optical retardation compensator plate (retardation compensator film) be inserted as an optical element having optical anisotropy between the liquid crystal display element and one of polarize plates.
According to the method, the elliptically polarized ray converted from a linearly polarized ray by passing through liquid crystal molecules having refractive index anisotropy is directed through the optical retardation compensator plates(s) disposed on the side(s) of the liquid crystal layer having refractive index anisotropy. Hence, the phase difference between the ordinary and extraordinary rays which occurs to the viewing angle are compensated for, and the elliptically polarized ray is converted back to the linearly polarized ray, which enables the restraint of the viewing angle dependency.
Japanese Laid-Open Paten Application No. 5-313159/1993 (Tokukaihei 5-313159), as an example, discloses an optical retardation compensator plates of the above kind represented by a refractive index ellipsoid with one of the principal refractive indices parallel to the normal to the surface of the optical retardation compensator plate. Nevertheless, this optical retardation compensator plate still cannot satisfactorily restrain the tone reversion phenomenon that occurs when the viewing angle increases in the standard viewing direction.
In order to eliminate the tone reversion phenomenon, Japanese Laid-Open Patent Application No. 57-186735/1982 (Tokukaisho 57-186835) discloses the so-called pixel dividing method, in which a displayed pattern (pixel) is divided and orientation is controlled so that each divided segment has its own viewing angle characteristics independent from those of the other segments. According to the method, since the liquid crystal molecules stand upwards in different directions from segment to segment, the viewing angle dependency can be eliminated. However, the problem of a lower contrast when the viewing angle increases upward or downward cannot be solved.
Japanese Laid-Open Patent Applications No. 6-118406/1994 (Tokukaihei 6-118406) and No. 6-194645/1994 (Tokukaihei 6-194645) disclose technologies to combine the pixel dividing method and an optical retardation compensator plate.
The liquid crystal display device disclosed in Japanese Laid-Open Patent Application No. 6-118406/1994 includes an optical anisotropic film (optical retardation compensator plate) interposed between the liquid crystal panel and the polarizer plate to, for example, improve the contrast. The retardation compensator plate (optical retardation compensator plate) disclosed in Japanese Laid-Open Patent Application No. 6-194645/1994 is set to have almost no phase difference in a plane parallel to the surface of the retardation compensator plate and to have a larger refractive index in a plane perpendicular to the surface of the retardation compensator plate than the refractive index in a plane parallel thereto, in order to have a negative refractive index. Therefore, when a voltage is applied, the positive refractive index occurring to the liquid crystal display element is compensated for and viewing angle dependency can be decreased.
Nevertheless, the application of the pixel dividing method to the use of this optical retardation compensator plate still fails to uniformly restrain the decrease in contrast in the vertical directions, coloration phenomenon still occurs in oblique directions when the viewing angle is 45xc2x0.
For these reasons, there are limits to the restraining of the contrast variations, coloration phenomenon, and tone reversion phenomenon related with viewing angle, by means of a retardation compensator plate represented by a refractive index ellipsoid positioned upright, i.e., a refractive index ellipsoid with one of the principal refractive indices thereof parallel to the normal to the surface of the retardation compensator plate.
Hence, Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) suggest the use of an optical retardation compensator plate represented by a refractive index ellipsoid with the principal refractive indices inclining to the normal to the surface of the optical retardation compensator plate. This method adopts two kinds of optical retardation compensator plates as follows.
One of the optical retardation compensator plates can be represented by such a refractive index ellipsoid that the smallest of the three principal refractive indices is parallel to the surface, one of the two larger principal refractive indices inclines to the surface of the optical retardation compensator plate by an angle xcex8, the remaining principal refractive index inclines to the normal to the optical retardation compensator plate by the same angle xcex8, and the angle xcex8 satisfies 20xc2x0xe2x89xa6xcex8xe2x89xa670xc2x0.
The other optical retardation compensator plate can be represented by a refractive index ellipsoid inclining to the surface, where the three principal refractive indices, na, nb, and nc, are mutually related by the inequality na=nc greater than nb, and the direction of the principal refractive index nb parallel to the normal to the surface and the direction of either the principal refractive index na or nc in the surface recline either clockwise or counterclockwise around the direction of the principal refractive index nc or na in the surface.
As for the former optical retardation compensator plate, a uniaxial and biaxial optical retardation compensator plate can be used. For the latter one, two optical retardation compensator plates, instead of one, can be used in such a combination that the two principal refractive indices nb form an angle of 90xc2x0.
A liquid crystal display device, incorporating at least one such optical retardation compensator plate between the liquid crystal display element and the polarizer plate exhibits some restraint in the contrast variations, coloration phenomenon, and tone reversion phenomenon caused by the viewing angle dependency of the display screen.
However, with today""s increasingly large demand on a wider effectual range of viewing angle and superb display quality, a better restraint in the viewing angle dependency is crucial. In this context, the optical retardation compensator plate disclosed in Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) above does not provide satisfactory solutions and needs to be improved.
In view of the above problems, the first object of the present invention is, on top of the improvement by the compensation effects by the optical retardation compensator plate, to restrain the viewing angle dependency, and especially, to effectively restrain the tone reversion in the opposite viewing direction when halftone is being displayed by applying a voltage that is close to the threshold voltage for the liquid crystal.
The second object of the present invention is, on top of the improvement by the compensation effects by the optical retardation compensator plate, to restrain the viewing angle dependency, and especially, to effectively restrain the coloration phenomenon.
In order to accomplish the first object, a liquid crystal display device of the first arrangement in accordance with the present invention includes:
a liquid crystal display element formed by sealing a liquid crystal layer between a pair of substrates;
a pair of polarizers disposed so as to flank the liquid crystal display element; and
at least one optical retardation compensator plate disposed between the liquid crystal display element and the polarizers, the optical retardation compensator plate being represented by an inclining refractive index ellipsoid,
wherein the pretilt angle formed by the orientation films and the longer axes of liquid crystal molecules in the liquid crystal layer is set within such a range that tone reversion does not occur in the opposite viewing direction when halftone is being displayed by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal.
As explained above, the first arrangement of the present invention incorporates, between the liquid crystal layer and the polarizer, an optical retardation compensator plate represented by an inclining refractive index ellipsoid. Therefore, with the arrangement, for a case where a linearly polarized ray is converted to an elliptically polarized ray according to the phase difference between the ordinary and extraordinary rays developed from the linearly polarized ray upon the passing through the liquid crystal layer possessing birefringence, the optical retardation compensator plate compensates for the phase difference between the ordinary and extraordinary rays that varies depending upon the viewing angle.
With the liquid crystal display device of the first arrangement in accordance with the present invention, the pretilt angle of the liquid crystal layer sealed in the liquid crystal display element is set within such a range that a tone reversion does not occur in the opposite viewing direction when halftone is being displayed by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal. This can eliminate the tone reversion in the opposite viewing direction on a screen displaying halftone, and thereby further restrain the viewing angle dependency of the screen. The contrast variations and coloration are also restrained better than only by the compensation function by the optical retardation compensator plate.
In the first arrangement above, the abrupt decrease in luminance can be restrained in the standard viewing direction when halftone is being displayed, by further setting the pretilt angle within such a range that luminance does not decrease abruptly in the standard viewing direction when halftone is being displayed by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal.
For these reasons, with the arrangement, the contrast ratio in black and white display is not affected by the viewing angle of the observer, and the quality of images displayed by the liquid crystal display device is greatly improved.
In order to accomplish the first object, a liquid crystal display device of the second arrangement in accordance with the present invention includes:
a liquid crystal display element formed by sealing a liquid crystal layer between a pair of substrates;
a pair of polarizers disposed so as to flank the liquid crystal display element; and
at least one optical retardation compensator plate disposed between the liquid crystal display element and the polarizers, the optical retardation compensator plate being represented by an inclining refractive index ellipsoid,
wherein the value of applied voltage for displaying halftone obtained by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal is set within such a range that tone reversion does not occur in the opposite viewing direction when halftone is being displayed.
Even if a linearly polarized ray is converted to an elliptically polarized ray according to the phase difference between the ordinary and extraordinary rays developed from the linearly polarized ray upon the passing through the liquid crystal layer possessing birefringence, the second arrangement compensates for the phase difference by the optical retardation compensator plate similarly to the first arrangement.
With the liquid crystal display device of the second arrangement, the value of applied voltage for displaying halftone obtained by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal is set within such a range that tone reversion does not occur in the opposite viewing direction when halftone is being displayed. This can eliminate the tone reversion in the opposite viewing direction with a screen displaying halftone, and thereby further restrain the viewing angle dependency of the screen. The contrast variations and coloration are also restrained better than only by the compensation function by the optical retardation compensator plate.
In the second arrangement above, the abrupt decrease in luminance can be restrained in the standard viewing direction when halftone is being displayed, by further setting the value of applied voltage for displaying halftone obtained by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal within such a range that luminance does not decrease abruptly in the standard viewing direction when halftone is being displayed.
For these reasons, with the arrangement, the contrast ratio in black and white display is not affected by the viewing angle of the observer, and the quality of images displayed by the liquid crystal display device is greatly improved.
In order to accomplish the second object, a liquid crystal display device of the third arrangement in accordance with the present invention includes:
a crystal display element formed by sealing a liquid crystal layer between a pair of substrates;
a pair of polarizers disposed so as to flank the liquid crystal display element; and
at least one optical retardation compensator plate disposed between the liquid crystal display element and the polarizers, the optical retardation compensator plate being represented by an inclining refractive index ellipsoid,
wherein the ratios of the variation in the refractive index anisotropy, xcex94nL, of the liquid crystal material for the liquid crystal layer with the wavelength of light and of the variation in the refractive index anisotropy, xcex94nF, of the optical retardation compensator plate with the wavelength of light are set within such a range that viewing angle dependency does not cause coloration on the liquid crystal screen.
Even is a linearly polarized ray is converted to an elliptically polarized ray according to the phase difference between the ordinary and extraordinary rays developed from the linearly polarized ray upon the passing through the liquid crystal layer possessing birefringence, the arrangement compensates for the phase difference by the optical retardation compensator plate, similarly to the first arrangement.
With the liquid crystal display device of the third embodiment, the ratios of the variation in the refractive index anisotropy, xcex94nL, of the liquid crystal material for the liquid crystal layer with the wavelength of light and of the variation in the refractive index anisotropy, xcex94nF, of the optical retardation compensator plate with the wavelength of light are set within such a range that viewing angle dependency does not cause coloration on the liquid crystal screen. This can further restrain coloration on the screen. The contrast variations and coloration are also restrained better than only by the compensation function by the optical retardation compensator plate.
Moreover, as described above, in the first, second, and third arrangements, the liquid crystal display device is preferably arranged so that the refractive index anisotropy, xcex94nL(550), of the liquid crystal material for the liquid crystal layer to light having a wavelength of 550 nm is set within a range larger than 0.060 and smaller than 0.120.
The setting can eliminate the phase difference that occurs to the liquid crystal display element in accordance with the viewing angle. That can further restrain the contrast variations and tone reversion phenomenon in the right- and left-hand directions, as well as the coloration phenomenon that occurs depending upon the viewing angle.
In such an event the phase difference that occurs to the liquid crystal display element in accordance with the viewing angle can be more effectively eliminated by setting the refractive index anisotropy, xcex94nL(550), of the liquid crystal material for the liquid crystal layer to light having a wavelength of 550 nm so as to be within a range not smaller than 0.070 and not larger than 0.095.
This can surely restrain the contrast variations and tone reversion phenomenon in the right- and left-hand directions of the images displayed by the liquid crystal display device.
Moreover, in the first, second, and third arrangements, the liquid crystal display device is preferably arranged so that the or each optical retardation compensator plate is represented by a refractive index ellipsoid inclining by an inclination angle xcex8 set within a range of 15xc2x0 to 75xc2x0.
By setting the inclination angle of the refractive index ellipsoid to be within a range of 15xc2x0 to 75xc2x0 with respect to the or each optical retardation compensator plate incorporated in the liquid crystal display device, it is assured that the present invention provides the aforementioned compensation function for the phase difference by the optical retardation compensator plate.
Moreover, in the first, second, and third arrangements, the liquid crystal display device is preferably arranged so that the or each optical retardation compensator plate has a product (naxe2x88x92nc)xc3x97d, of the difference between the principal refractive indices, na and nb, and the thickness, d, of the optical retardation compensator plate, the product being set to be from 80 nm to 250 nm.
By setting the product, (naxe2x88x92nb)xc3x97d, of the difference between the principal refractive indices, na and nb, and the thickness, d, of the optical retardation compensator plate, so as to be from 80 nm to 250 nm with respect to the or each optical retardation compensator plate incorporated in the liquid crystal display device, it is assured that the present invention provides the aforementioned compensation function for the phase difference by the optical retardation compensator plate.